Audio line-up levels and different meter scales

You will come across many phrases along the lines of "Line up tone at zero
level" or "Set the 1KHz tone to zero dB", but what exactly do they mean?
Well, without some further quantifying, neither of those phrases define an absolute level,
and are likely to mean different things to different people in different contexts. So how
do you find your way through this minefield of audio levels?

Firstly, you should be careful about how the term dB is used, because much of the
confusion comes from the casual misuse of dB to describe reference levels. Zero dB is NOT
a reference level! Decibels (dBs) are simply a logarithmic measure of relative power,
where +10dB equates to a tenfold increase in power, +3dB equals double power, and zero dB
means no change in power, or equal in power. So you can see that the term "zero
dB" can only have an absolute (rather than relative) value if we know to what the
comparison is being made.

In the early days of audio communications, a standard level of 1mW into a 600 ohm line
was adopted. The compromise was between higher levels requiring higher power consumption,
and lower levels which were more prone to induced and thermal noise. This level was
referred to as "zero dBm" (0dB = 1mW/600ohm), so signals could be measured
relative to this power level and the results expressed in dBm. In voltage terms, 0dBm =
0.775Vrms (Volts - Root Mean Squared), and this is still a common analogue measurement
reference today. Because many audio circuits don't always drive 600 ohm lines (e.g. higher
impedances require less power at the same voltage), the more common reference standard is
actually dBv7, where the "v7" means 0.775Vrms into any impedance, also
known as dBv or dBu.
There's also a dBV reference (0dBV - 1Vrms), so the introduction of dBu made it
easier to seperate between big 'V' and small 'v' reference levels. However, you will very rarely see "dBv7" in writing,
as dBu is
more commonly used, and it's not that unusual to see dBm (mis-)used to mean dBv7.

So how does this relate to audio level meters? Well, yet more standards, I'm afraid.
One of the most common electro-mechanical meters is the Bell Lab's VU, which measures
Volume Units. Here, the zero level (0VU) is set at 1.23Vrms, which equates to +4dBu, and the
difference between volume unit values is equivalent to dBs (e.g. 0VU is 4dB
higher than -4VU). So from this you can see
that 0dBu is equal to -4VU, which you may also see written as -4dBVU or
-4dBvu).

In
the UK one of the most common meter types in professional use is the BBC-style PPM. This
has no dB values indicated, and instead has equally spaced markers labelled from one to
seven, with number four at dead centre. Four on a PPM is equivalent to
the reference level 0dBm (or 0dBu), and
the rest of the scale from 1 to 7 is in 4dB increments. So, for example, PPM 6 = +8dBu, and PPM 1 =
-12dBu. Programme material is usually limited to peak at PPM 6 (+8dBu).

When it comes to recording devices, which used to be exclusively tape-based, reference
levels related to magnetic flux levels (measured in nWb/m - nano-Webers/metre - or
pWb/mm), and to how much higher you could go without distortion. There were slightly
different standards from America and Europe, including NAB, IEC & CCIR. Anyone
remember the excellent Hugh Ford, and his Studio Sound reviews of tape machines, including
what level (in dBm) you could expect to get out when playing back a tape with the a flux
level of 320pWb/mm? Depending on the type of tape, and recording speed, you could
typically push the levels from 6-12dB above the reference flux before distortion started
to rise more rapidly.

Digital systems behave quite differently, however, and here the distortion decreases
right up to the point of saturation, after which hard (and nasty sounding) peak clipping
occurs. Unlike brands of tape, all digital systems behave in exactly the same way in this
respect, and so it is common for meters on digital systems to use the clipping point
itself as a reference, which is known as dBfs (meaning dBs referred to Full Scale).
Clearly a reference level which represents the onset of digital clipping can't be
equivalent to analogue line or tape-based flux levels which are all set below the onset of
distortion, so what is the conversion?

The EBU in Europe and NAB in America came up with slightly different recommendations
(as ever), such that the EBU recommends 0dBu = -18dBfs (= PPM 4), and the NAB has 0VU =
-20dBfs (or 0dBu = -24dBfs). Peak levels are therefore expected to be about 8-9dB higher at
around -10dBfs. The extra 10dB headroom is for fast transients that meters, operators and
even compressor/limiters don't always catch. Armed with this information, you might be
able to make more sense of the audio line-up jungle, and at least ask the right questions
when someone says "zero dB", like, "Referenced to what?".

Recording HDV as Matrox AVIs in Premiere Pro CS4
(with the RT.X2)

With some combinations of HDV cameras/decks and Vista 64 & Windows 7,
Premiere fails to recognise the camera as a suitable HDV source when in "Matrox
mode", even though HDV can be captured using Adobe's native HDV capture mode. If
you're experiencing this problem, here's a possible solution from Linda Rossiter
(www.rossiterandco.com)

a): Turn on camera and ensure settings are correct
b): Start playing a HDV tape
c): Plug in firewire cable and wait for Windows to detect the camera, which
should appear in Device Manager as a Microsoft AV/C Tape Subunit Device

VT Ident Clocks, and other TV history

Editing Multi-Camera footage with Digisuite/Speed Razor

Most low to mid range NLE systems are not geared to cutting multi-camera footage, Speed
Razor included, but its freeform-style timeline lends itself to a useful technique in
conjunction with a real-time board like the Matrox Digisuite.Let's say you have an event
which has been covered by three cameras, one static with the master sound mix, and two
mobile units which have additional sync sound and effectively a guide track of the main
mix. Of course you can cut this with any NLE software and any video board, but the
advantage of the technique described here is that you don't have to cut any of the video
tracks, which makes it easier to keep sync between the various camera tracks. Here's what
you do.

Firstly, lay down the main video track with the master audio track, and then below it
add the two mobile cameras' video tracks with their sync audio/ guide tracks. Turn on
"show audio waveforms" and zoom right in on the timeline whilst you slide the
various tracks to achieve sync. Once you have the audio tracks in sync, turn off waveforms
if not required, and do a rough audio mix (or mute the guide tracks) to leave a
"working" audio track mix. Arrange the order of the video clips so you have the
one you're going to use most often on the highest numbered video track (so you'll see it
as the "default" video track). In this example you now have video on tracks 1, 2
& 3.

Now the editing. Play the video until you want to cut away from the "default"
camera and hit Tab to add a marker at that point. Then add the MatroxDVE effect onto track
5 (not 4, or else it'll extend and auto-attach itself to the video on track 3). It's a
good idea also to turn off "Change Effects Parameters when drop into
Composition" in Misc Preferences, as you don't want to change the DVE effect
parameters most of the time. Instead, just drag it so that the start lines up with your
marker, extend the other end as far down the timeline as possible, and source it to
whichever video track you want to cut to (let's say track 1). Leave the effect
highlighted, move your "current position" marker back to before the edit and hit
play. At the marked point the DVE on track 5 will take priority over the video on track 3
and do a full-size DVE of the track 1 video over the top of it - effectively a cut, but
without cutting any actual video tracks. As the cut-away plays, hit Stop (spacebar) where
you want the shot to end, and press 'O' to shorten the DVE effect (which you left
highlighted) and set its Outpoint to the current position. This only works if the effect
is already longer, rather then shorter, than the desired length.

Of course sometimes the decision on where to make the outpoint is based on what's
happening on the original video track, rather than on the cutaway. In that case
temporarily change the function of the DVE to create a small Picture In Picture so you can
see the background video whilst the edit plays. Set the DVE back to "full
screen" after you're happy with the edit points. Or just place two markers (for In
& Out) when you played the original video track and stretch the DVE to fit between
them.

This technique can be applied to any number of synchronous video tracks, with the
advantage that all the sync audio tracks are always available without having to
"separate audio and video", and without the risk of chopped-up video fragments
getting nudged out of sync by accidental moving when attempting to trim. Also, you can
"invisibly" remove & repair an edit just by removing the associated Matrox
DVE effect.

Nothing's perfect, of course, and there are limitations to this technique, particularly
if you need transitions between shots. In the above example, there's no problem with
transitions between V3 & V1, or V3 & V2; just place the MatroxTrans on track 6 and
source either the In or Out (as appropriate) to the DVE effect on track 5. It's when you
want a transition between V1 & V2 that the problem arises, because you can only use
one DVE over the stack of video tracks without having to render, and you would need two in
order to get both V1 and V2 over the top of V3, without physically cutting and moving
them, or cutting a hole in V3, which is what we're trying to avoid. Still, it works most
of the time and for cuts-only editing I've yet to find a quicker or more versatile method
with this level of software.

Obscure (and forgotten?) bugs in Speed Razor

Here's one for starters that's been around since version 3 and is still with us in
version 4.7. It's not fatal, but it can cause a fair amount of panic and confusion amongst
the unwary. I call it the Directory Level bug, and it works like this. In the
Preferences->Directories menu, you have a dialogue box in which you specify paths for
various data like Video Finals, Audio Finals, Projects, etc. To these paths SR adds the
name of your project when you start a New Project, in order to create a new set of
directories to store your data. Often, the path may be to the root of a disk (like V:\ for
video files), but in the case of project (.rzx) files it defaults to
"C:\razor47\Projects\". When you start a new project (e.g. Showreel) you get a
new directory called "C:\razor47\Projects\Showreel" into which your project
files will be saved.

All well and good until for some reason you start up your system without one of the
specified disk drives. This can easily happen if you have external SCSI drives and you
forget to power them up before starting the PC. You run Speed Razor and get a message
about it not finding or creating a directory on one of your drives, and you have to change
the appropriate Video/Audio Final path before you can continue. Probably you quit and
reboot with the drives powered up, run Razor again, and re-add the drive back into the
system. What you probably won't have noticed is that every time SR starts and can't find a
disk, it bumps all the Directory Paths up a level - if they aren't already set to a root
level, that is. So, in the example above, "C:\razor47\Projects\" has been
shortened to "C:\razor47" , and a new directory has been created at
"C:\razor47\Showreel". You won't notice if you load your project via the
"recently saved" list at the bottom of the File menu, but next time you select
"Save As" you'll find an empty directory and wonder where all your previous
project files have gone.

Windows doesn't help much here, because the top line of the File Dialogue box only
shows the current directory (which is still Showreel) but not the whole path, so you don't
see that the path has changed and that you've gained an extra directory at a different
level of the hierarchy. Also, the auto-numbering gets screwed up too, and the suggested
version number resets to zero because it doesn't find any previous versions there. If this
scenario happens several times, you end up with a succession of directories at all the
available levels, ending with C:\Showreel, which potentially might contain both project
files (which started in C:\razor47\Projects\Showreel) and audio (which started in
C:\razor47\audio\Showreel). If you check your system and find you have several
"Default" directories at different levels on your C drive, then that's probably
how they got there!

Lighting tips for ChromaKey shoots

Now I'm no Lighting Director or DOP, but I've been on enough ChromaKey shoots to know
the important role that lighting plays in getting good results at the post production
stage. And the first thing to realise is that you really need quite a few lights and
plenty of space. The two main enemies of good ChromaKey are uneven background lighting,
and reflected spill onto the actors or foreground objects.

Start by lighting the background, as evenly as possible, with no shadows. These lights
should be behind, or at worst level with, the foreground action. Then light the foreground
with its own set of lights. Space plays a big part here, because if you can get lots of
separation between the actors and the coloured background, then there's less chance that
the foreground shadows will hit the backing. Also, distance will minimise the "colour
fringe" from light reflected off the backing.

Finally, to neutralise any remaining colour-cast from light reflected off the backing,
add some soft back-light from both sides, using a gel of a complementary colour to the
backing. So for a blue backing, use CTO (orange) backlight, in the same way that you would
match the different colours of daylight and tungsten lights. Adjust the level (or
distance) of the backlights until you have neutralised the reflected spill.

And that's it! If you come into post production with blue-screen footage
where the background is a nice even blue, and there's no "blue fringe" on the
clothes, hair and skin, then even the most basic ChromaKey hardware or software will stand
a chance of giving you good results. If not, and it's another lengthy (expensive) rescue
job....

If you want to contribute to our support page, or if you think
that you have a workaround that may be of use to someone, then please
Email us with the details.

" I've been a regular contributor to user groups and technical forums for
the last 20 years or so, and have written hundreds of posts
offering advice and information on a whole range of computer, audio &
video production topics."

What we're about . . .
ZEN is not a traditional Audio-Visual dealer who started selling computers,
nor is it a computer shop that also sells video products. You won't
get any salesmen giving you the "hard-sell" when you call, just
straightforward advice and information - which for some callers is the
knowledge that they don't need to buy whatever it is they thought they
needed! Above all you'll be dealing with someone with a wide range of
experience and knowledge of both PCs and video production. We're not the
biggest, nor necessarily the cheapest, but we are one of the longest
established computer/video specialists in the UK.

Company history . . .
ZEN was started in the 1980s by Martin Kay, then working for ITV at
Granada's Manchester studios, who built his first 6502-based computer in
1979 from an Ohio Scientific kit, bought in the USA whilst working as a
Sound Recordist on a
film shoot for World In Action. With the advent of the Amiga, which could be gen-locked
to a video source, Martin started writing a variety of video-related software. This
included subtitling & tele-prompting, ident clocks, scoring software for
sports & gameshows, and specialist software to mimic other computer
displays for
use in TV film dramas like Cracker, Prime Suspect and A Touch of Frost.
Martin left Granada in 1993 to concentrate on his computer-video activities
with ZEN, following a natural path into non-linear editing systems,
for many years the main business activity, although he still maintains an active
interest in video production.